Abnormal grain growth in ultrafine grained metals under high cycle loading

大多数在技术上很重要的金属是多晶的，由许多称为颗粒的小团簇组成。这些颗粒的大小是一个关键参数，因为颗粒大小对材料的强度有很大影响。同样的金属，如果它的颗粒更小，就会更坚固。因此，晶粒度小于1微米的超细晶粒金属是一类非常重要的结构材料，因为它们具有特别高的强度，在航空航天和核工业等领域具有关键应用。细晶金属的另一个显著优点是，它们往往具有更好的疲劳性能，在循环载荷下不太可能形成有害的裂纹。因此，在超细晶粒结构金属的整个生命周期中保持较小的晶粒度是至关重要的，否则其机械性能可能会灾难性地退化。在这个项目中，PIs正在发展对颗粒生长机理的新理解，以便能够适当地控制颗粒尺寸。PI特别关注异常谷物生长，即与其他谷物相比，一小部分谷物生长得非常大和快，因此消耗了其他谷物。虽然这种现象在高温和高应变下很好地被理解，但对于在很少探索的高周加载范围内(在室温下以低应变进行大量循环)的异常晶粒生长却知之甚少。该项目的外展活动包括一个材料科学和工程的暑期充实计划，目标是STEM领域中代表性不足的群体的高中生，并让高中教师、研究生和本科生参与制定课程和实施该计划。技术总结这项建议的首要目标是从根本上了解超细晶金属在室温高周加载下异常晶粒长大的原因。这一假设的中心假设是，在室温高周加载条件下，弹性各向异性效应主导了晶粒长大的驱动力，导致了异常的晶粒长大行为。PI通过在配备有电子背散射衍射的扫描电子显微镜内高通量表征循环载荷诱导的超细晶粒金属中的晶粒生长来验证这一假说。制备和测试了六种具有不同程度弹性各向异性的金属薄膜，其中面心立方和体心立方结构是本工作的重点。这些实验可以表征不同应变幅度(高达1%)下，晶粒度分布和取向随施加循环的变化规律。PI使用细观力学和相场模拟来确定热力学驱动力，即应变能密度和异常晶粒生长的晶界迁移率。综合实验和模型被用来确定控制室温下面心立方和体心立方金属中异常颗粒生长的主要因素和加载范围，包括其动力学。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARYMost technologically important metals are polycrystalline, and are made of many small clusters called grains. The size of these grains is a key parameter, as grain size has a strong influence on a material’s strength. The same metal is much stronger if its grains are smaller. For this reason, ultrafine grained metals, with grain sizes less than one micrometer, are a very important class of structural materials due to their particularly high strength, with critical applications in the aerospace and nuclear industry, among others. Another significant advantage of small grained metals is that they tend to have better fatigue properties and are less likely to form detrimental cracks under cyclic loading. It is therefore crucial to keep small grain sizes throughout the lifetime of an ultrafine grained structural metal; otherwise its mechanical properties can degrade catastrophically. In this project, the PIs are developing new understanding of the mechanics of grain growth, so that grain size can be properly controlled. The PIs focus specifically on abnormal grain growth, where a small fraction of grains grow drastically large and fast compared to other grains and as a result consume other grains. While this phenomenon is well understood at high temperatures and high strains, little is known about abnormal grain growth in the rarely explored range of high cycle loading (applying a large number of cycles with low strain at room temperature). The outreach activities in this project include a summer enrichment program in material science and engineering, targeting high school students from underrepresented groups in the STEM fields, and involving high school teachers, graduate and undergraduate students to develop the curriculum and implement the program. TECHNICAL SUMMARYThe overarching goal of this proposal is to achieve a fundamental understanding of the origins of abnormal grain growth in ultrafine grained metals under high-cycle loading at room temperature. The central hypothesis of this proposal is that the elastic anisotropy effect dominates the driving force for grain growth in the high-cycle loading regime at room temperature, resulting in abnormal grain growth behavior. The PIs test this hypothesis through high-throughput characterization of cyclic-load-induced grain growth in ultrafine grained metals inside a scanning electron microscope equipped with electron back scattered diffraction. Fabrication and testing of six different metallic films with varying degrees of elastic anisotropy, with face-centered cubic or body-centered cubic structures are the focus of the work. These experiments can characterize the evolution of grain size distribution and orientation as a function of applied cycles, for various strain amplitudes (up to 1%). The PIs use micromechanics and phase field modeling to determine the thermodynamic driving forces in terms of strain energy densities and the grain boundary mobilities for abnormal grain growth. The integrated experiments and modeling are being used to identify the predominant factors and loading ranges controlling abnormal grain growth, including its kinetics, in face-centered cubic and body-centered cubic metals at room temperature.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.